The electrical energy production is at the moment going through major changes. The pollution and greenhouse gas emissions of the energy sector have gained increasing attention. At the same time as the electrical energy production is moving towards renewable energy based energy production, the electrical grid is also facing new challenges. Previously, the power plants connected to the electrical grid were very large such as nuclear power plants, large coal-based power plants, etc. This centralized electrical power generation, of course, causes losses in the electrical grid because the energy must be transferred over long distances.
Distributed power generation is closer to the consumption and thus smaller losses occur in the electrical grid due to shorter distances over which the energy is being transferred compared to the case of centralized power generation. In contrast to the centralized power generation plants which typically operate at their rated power, the distributed power generation plants have to be able to constantly adjust their operation and output power based on load demand. This is especially important if the electrical grid seizes to feed or receive power such as in case of islanding conditions during which the power generation and power consumption must be equal typically in a rather small area wherein the electrical grid is weak. In these cases, the operation is entirely relying on the control and operation of a single power generation unit or few power generation units. It is, therefore, of utmost importance to have power plants which can run at high efficiency also at part-load conditions and can adjust their output rapidly.
One major reason why the amount of distributed energy production has not increased more is the higher price of the energy produced by these systems compared to, e.g., price of the electricity from the grid. This is typically due to a lower electrical efficiency compared to large power plants. Gas engine or gas turbine plants, which are more and more being used in the electrical energy production, are good examples of power plants that can be utilized in distributed power generation.
Gas turbine plants are typically designed to operate at 100 percent of the nominal load, i.e. the design point. Nowadays, the electrical production efficiencies of commercial gas turbines at their design points are at the most around 40 percent, especially in the plants with electrical power rating less than 20 megawatts at most. The electrical efficiency which itself is not very high, quickly decreases if the gas turbine is being operated at part-load conditions, i.e., at load conditions less than 100 percent of the nominal load.
A typical gas turbine power plant comprises a compressor, a combustor, a turbine and an electrical generator. The compressor and the turbine are mounted on the same shaft and form a single spool. The generator is also mounted on the shaft. Some prior art, however, describes solutions with gas turbines having two spools. Two-spool arrangement offers potentially better efficiency than a single-spool system because more power can be produced with the same turbine inlet temperature compared to a single-spool arrangement.
Some prior art describes also gas turbines with multi-spool arrangement. Most of these are in the aviation related applications in which the weight and compactness are very important in the designing of these systems. In land- and marine-based applications, the size and structure are less important but, on the other hand, the efficiency and controllability become more important. Also, especially, in distributed power generation, the controllability and part-load operation are essential when designing the gas turbine plant.
Most commonly, multi-spool gas turbines in land-based applications have two spools. The two spools of the gas turbines are different in a way that there is a high pressure spool and a low pressure spool. Low pressure spool is typically connected to the main electrical generator while the high pressure spool is operating as a gas compressing spool. The magnitude of the pressure increase that a compressor in a single-spool system or the two compressors in the two-spool system must be able to produce typically affects the efficiency of the compressors and the system in a way that the higher the total pressure increase, the lower the compressor efficiency.
In some attempted solutions of the gas turbine plants, two or more spools have been utilized wherein both or all of the spools have electrical generators coupled to the spools. In these solutions, the power taken out of the gas turbine plant has been taken mainly from a single electrical generator, that is a main generator, and the other generators have been working as auxiliary motors/generators, typically having lower power ratings than the main generator and having rotational speeds at different speed ranges than the main generator. There are also solutions in which both or all of the electrical generators have been used primarily for controlling the operation of the gas turbine plant, thus both or all of the generators being auxiliary motors/generators, while the power taken out of the gas turbine plant is mainly taken from an additional free turbine spool to which an additional generator, operating in these cases as the main generator, is connected to.